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<liclass="toctree-l1"><aclass="reference internal" href="../intro.html">Python Like You Mean It</a></li>
<liclass="toctree-l1"><aclass="reference internal" href="../module_1.html">Module 1: Getting Started with Python</a></li>
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<divclass="section" id="Scope">
<h1>Scope<aclass="headerlink" href="#Scope" title="Permalink to this headline"></a></h1>
<p>A valuable aspect of the “encapsulation” provided by functions is that the function’s input argument variables, and any variables defined within the function, cannot be “seen” nor accessed outside of the function. That is, these variables are said to have a restricted <strong>scope</strong>.</p>
<divclass="admonition note">
<pclass="admonition-title fa fa-exclamation-circle"><strong>Definition</strong>:</p>
<p>The <strong>scope</strong> of a variable refers to the context in which that variable is visible/accessible to the Python interpreter.</p>
</div>
<p>Until our work with comprehension-statements and functions, we had only encountered variables that have <strong>file scope</strong>. This means that a variable, once defined, is visible to all parts of the code contained in the same file. Variables with file scope can even be accessed <em>within functions</em>. By contrast, the variables defined within a function or as input arguments to a function have a <strong>restricted scope</strong> - they can only be accessed within the context of the function:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">3</span><spanclass="c1"># `x` has file scope. It can be even be accessed</span>
<spanclass="c1"># within a function, even if it isn't passed to</span>
<spanclass="c1"># the function as an argument</span>
<spanclass="c1"># `my_func` has file scope (after it is defined)</span>
<spanclass="k">def</span><spanclass="nf">my_func</span><spanclass="p">(</span><spanclass="n">y</span><spanclass="p">):</span>
<spanclass="n">func_var</span><spanclass="o">=</span><spanclass="mi">9</span><spanclass="o">+</span><spanclass="n">x</span><spanclass="c1"># `x` will have the value 3</span>
<spanclass="c1"># the scope of `y` and `func_var` is restricted to this function</span>
<spanclass="k">return</span><spanclass="n">y</span>
<spanclass="c1"># `func_var` and `y` do not exist here</span>
<spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">func_var</span><spanclass="p">)</span><spanclass="c1"># raises NameError: name `func_var` not defined</span>
<spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">y</span><spanclass="p">)</span><spanclass="c1"># raises NameError: name `y` not defined</span>
</pre></div>
</div>
<p>Python’s scoping rules are quite liberal compared to those of other languages, like C++. In most situations, Python will give variables file scope. Let’s briefly survey the various contexts in which variables are defined, along with their corresponding scoping rules. Assume that the following code represents the entire contents of the Python script “example_scope.py”:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="c1"># this demonstrates scope of variables in different contexts</span>
<spanclass="c1"># nothing meaningful is computed in this file</span>
<spanclass="kn">from</span><spanclass="nn">itertools</span><spanclass="kn">import</span><spanclass="n">combinations</span><spanclass="c1"># `combinations` has file scope</span>
<spanclass="c1"># `my_func` has file scope</span>
<spanclass="c1"># `in_arg1` has restricted scope</span>
<spanclass="c1"># `in_arg2` has restricted scope</span>
<spanclass="c1"># `func_block` has restricted scope</span>
<spanclass="k">def</span><spanclass="nf">my_func</span><spanclass="p">(</span><spanclass="n">in_arg1</span><spanclass="p">,</span><spanclass="n">in_arg2</span><spanclass="o">=</span><spanclass="s2">"cat"</span><spanclass="p">):</span>
<spanclass="n">func_block</span><spanclass="o">=</span><spanclass="mi">1</span>
<spanclass="k">return</span><spanclass="kc">None</span>
<spanclass="c1"># `file_var` has file scope</span>
<spanclass="c1"># `comp_var` has restricted scope</span>
<spanclass="n">file_var</span><spanclass="o">=</span><spanclass="p">[</span><spanclass="n">comp_var</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="k">for</span><spanclass="n">comp_var</span><spanclass="ow">in</span><spanclass="p">[</span><spanclass="o">-</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="o">-</span><spanclass="mi">2</span><spanclass="p">]]</span>
<spanclass="c1"># `if_block` has file scope</span>
<spanclass="k">if</span><spanclass="kc">True</span><spanclass="p">:</span>
<spanclass="n">if_block</span><spanclass="o">=</span><spanclass="mi">2</span>
<spanclass="k">else</span><spanclass="p">:</span>
<spanclass="n">if_block</span><spanclass="o">=</span><spanclass="mi">3</span>
<spanclass="c1"># `it_var` has file scope</span>
<spanclass="c1"># `for_block` has file scope</span>
<spanclass="k">for</span><spanclass="n">it_var</span><spanclass="ow">in</span><spanclass="p">[</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">]:</span>
<spanclass="n">for_block</span><spanclass="o">=</span><spanclass="mi">1</span>
<spanclass="c1"># `while_block` has file scope</span>
<spanclass="k">while</span><spanclass="kc">True</span><spanclass="p">:</span>
<spanclass="n">while_block</span><spanclass="o">=</span><spanclass="kc">None</span>
<spanclass="k">break</span>
</pre></div>
</div>
<p>In the preceding code, the following variables have <em>file scope</em>:</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">combinations</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">my_func</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">file_var</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">if_block</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">it_var</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">for_block</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">while_block</span></code></p></li>
</ul>
<p>whereas the following variables have <em>restricted scope</em>:</p>
<ulclass="simple">
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">in_arg1</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">in_arg2</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">func_block</span></code></p></li>
<li><p><codeclass="docutils literal notranslate"><spanclass="pre">comp_var</span></code></p></li>
</ul>
<p>In C++, the variables <codeclass="docutils literal notranslate"><spanclass="pre">if_block</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">it_var</span></code>, <codeclass="docutils literal notranslate"><spanclass="pre">for_block</span></code>, and <codeclass="docutils literal notranslate"><spanclass="pre">while_block</span></code> all would have had restricted scopes - these variables would not be defined outside of their respective if/for/while blocks.</p>
<divclass="admonition note">
<pclass="admonition-title fa fa-exclamation-circle"><strong>Takeaway</strong>:</p>
<p>Variables defined within a function have a <em>restricted scope</em> such that they do not exist outside of that function. Most other contexts for defining variables in Python produce variables with <em>file scope</em> (i.e. they can be accessed anywhere in the file’s code, subsequent to their definition).</p>
</div>
<divclass="section" id="Variable-Shadowing">
<h2>Variable Shadowing<aclass="headerlink" href="#Variable-Shadowing" title="Permalink to this headline"></a></h2>
<p>What happens when a file-scope variable and a function-scope variable share the same name? This type of circumstance is known as <strong>variable shadowing</strong>. Python resolves this by giving precedence to the variable <em>with the most restricted scope</em>, when inside that scope:</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="n">x</span><spanclass="o">=</span><spanclass="mi">2</span>
<spanclass="n">y</span><spanclass="o">=</span><spanclass="mi">3</span>
<spanclass="k">def</span><spanclass="nf">func</span><spanclass="p">(</span><spanclass="n">x</span><spanclass="p">):</span>
<spanclass="c1"># input-arg `x` overrides file-scope version of `x`</span>
<spanclass="n">y</span><spanclass="o">=</span><spanclass="mi">5</span><spanclass="c1"># overrides file-scope version of `y`</span>
<spanclass="k">return</span><spanclass="n">x</span><spanclass="o">+</span><spanclass="n">y</span>
<spanclass="c1"># `x` is 2 here, once again</span>
<spanclass="c1"># `y` is 3 here, once again</span>
<spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">func</span><spanclass="p">(</span><spanclass="o">-</span><spanclass="mi">5</span><spanclass="p">))</span><spanclass="c1"># prints 0</span>
<spanclass="nb">print</span><spanclass="p">(</span><spanclass="n">x</span><spanclass="p">,</span><spanclass="n">y</span><spanclass="p">)</span><spanclass="c1"># prints 2 3</span>
</pre></div>
</div>
<p>and similarly,</p>
<divclass="highlight-python notranslate"><divclass="highlight"><pre><span></span><spanclass="n">it</span><spanclass="o">=</span><spanclass="s2">"cow"</span>
<spanclass="k">def</span><spanclass="nf">func</span><spanclass="p">():</span>
<spanclass="n">it</span><spanclass="o">=</span><spanclass="s2">"dog"</span><spanclass="c1"># overrides file-scope version of `it`</span>
<spanclass="n">my_list</span><spanclass="o">=</span><spanclass="p">[</span><spanclass="n">it</span><spanclass="o">**</span><spanclass="mi">2</span><spanclass="k">for</span><spanclass="n">it</span><spanclass="ow">in</span><spanclass="p">[</span><spanclass="mi">1</span><spanclass="p">,</span><spanclass="mi">2</span><spanclass="p">,</span><spanclass="mi">3</span><spanclass="p">]]</span><spanclass="c1"># within the list comprehension, the func-scope `it` is overridden</span>
<spanclass="c1"># `it` is "dog" here, once again</span>
<spanclass="k">return</span><spanclass="kc">None</span>
<spanclass="c1"># `it` is "cow" here, once again</span>
</pre></div>
</div>
</div>
<divclass="section" id="Links-to-Official-Documentation">
<h2>Links to Official Documentation<aclass="headerlink" href="#Links-to-Official-Documentation" title="Permalink to this headline"></a></h2>
<ulclass="simple">
<li><p><aclass="reference external" href="https://docs.python.org/3/tutorial/classes.html#python-scopes-and-namespaces">Scopes and namespaces</a></p></li>
<li><p><aclass="reference external" href="https://docs.python.org/3/reference/executionmodel.html">Python’s execution model</a></p></li>
</ul>
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